Teachers' notes and lesson plans - University of Southampton

Handout for lesson 2STAGE 1 – P is the commonest letter in the ciphertext so could stand for e - maybethe first word is the:the HYXHLtHTe MO AWFJYFLT H RFNheS HJNhHAet FL thFU ZHCFU thHt Ft FU eHUC tM KeKMSFUe the IeCZMSY MS IeCNhSHUe,HLY heLRe the RFNheS HJNhHAet. thFU FU FKNMStHLt, AeRHWUeFO the UeLYeS hHU tM IeeN the RFNheS HJNhHAet ML H NFeReMO NHNeS, the eLeKC RHL RHNtWSe the NHNeS, YFURMXeS theIeC, HLY SeHY HLC RMKKWLFRHtFMLU thHt hHXe AeeL eLRSCNteYZFth Ft. hMZeXeS FO the IeC RHL Ae RMKKFtteY tM KeKMSC FtFU JeUU JFIeJC tM OHJJ FLtM eLeKC hHLYU.STAGE 2 We see the single letter word H, and the four letter word thHt in the firstline - guess that H encodes the letter a.the aYXaLtaTe MO AWFJYFLT a RFNheS aJNhaAet FL thFU ZaCFU that Ft FU eaUC tM KeKMSFUe the IeCZMSY MS IeCNhSaUe,aLY heLRe the RFNheS aJNhaAet. thFU FU FKNMStaLt, AeRaWUeFO the UeLYeS haU tM IeeN the RFNheS aJNhaAet ML a NFeReMO NaNeS, the eLeKC RaL RaNtWSe the NaNeS, YFURMXeS theIeC, aLY SeaY aLC RMKKWLFRatFMLU that haXe AeeL eLRSCNteYZFth Ft. hMZeXeS FO the IeC RaL Ae RMKKFtteY tM KeKMSC FtFU JeUU JFIeJC tM OaJJ FLtM eLeKC haLYU.STAGE 3 The two 2 letter words Ft FU are probably it is meaning that Fencodes i and U encodes s:the aYXaLtaTe MO AWiJYiLT a RiNheS aJNhaAet iL this ZaCis that it is easC tM KeKMSise the IeCZMSY MS IeCNhSase,aLY heLRe the RiNheS aJNhaAet. this is iKNMStaLt, AeRaWseiO the seLYeS has tM IeeN the RiNheS aJNhaAet ML a NieReMO NaNeS, the eLeKC RaL RaNtWSe teh NaNeS, YisRMXeS theIeC, aLY SeaY aLC RMKKWLiRatiMLs that haXe AeeL eLRSCNteYZith it. hMZeXeS iO the IeC RaL Ae RMKKitteY tM KeKMSC itis Jess JiIeJC tM OaJJ iLtM eLeKC haLYs.STAGE 4: haXe = have, easC = easy and so on - we get the following extractfrom Simon Singh’s excellent history of codes and ciphers, The Code Book:“The advantage of building a cipher alphabet in this way is that it is easy to memorisethe keyword or keyphrase, and hence the cipher alphabet. This is important, becauseif the sender has to keep the cipher alphabet on a piece of paper, the enemy cancapture the paper, discover the key, and read any communications that have beenencrypted with it. However if the key can be committed to memory it is less likely tofall into enemy hands."26